The subject invention relates generally to a heat exchanger having a plurality of refrigerant tubes extending between an inlet header and an outlet header for use with a two phase refrigerant undergoing a liquid to vapor transformation; more particularly to an improved refrigerant collector conduit disposed in the outlet header for uniformly collecting the vapor phase of the refrigerant.
Due to their high performance, automotive style brazed heat exchangers can be modified for residential and commercial air conditioning and heat pump applications. Automotive heat exchangers typically utilize a pair of manifold headers with multi-port extruded tubes defining fluid passages that interconnect the manifold headers. Corrugated air fins interconnect the tubes for improved heat transfer between the extruded tubes and ambient air. In modified automotive heat exchangers for residential applications, uniform refrigerant distribution through the manifolds and extruded tubes is necessary for optimal performance.
Modified automotive style brazed heat exchangers can be used as indoor and outdoor heat exchanger coils in residential and commercial air conditioning and heat pump systems. In cooling mode the indoor heat exchanger coil acts as the evaporator. In heating mode the outdoor heat exchanger coil acts as the evaporator. The substantially vertical refrigerant tubes interconnecting the substantially horizontal manifold headers of the automotive style heat exchanger form the core of the heat exchange coil. During operation in evaporative mode, partially expanded two phase refrigerant enters the lower portions of the refrigerant tubes where it continues to expand, absorbing heat from the air as it rises within the tube and changing into a vapor phase. Momentum and gravity effects due to the large mass differences between the liquid and gas phases can result in separation of the phases within the manifold and cause poor refrigerant distribution throughout the refrigerant tubes. This degrades evaporator performance and can result in hot spots over the core and in low temperature heating mode can result in increased icing or frosting of the core.
The increase in length requirement of the manifold header for residential and commercial as compared to automotive use dramatically increases the length of the manifold header where the two phase refrigerant needs to remain mixed without allowing the liquid to separate. Distributor tubes are used to obtain better refrigerant distribution in the inlet manifold header. These inlet distributors are intended to deliver partially expanded two phase refrigerant uniformly along their length. An example of such a heat exchanger having a refrigerant conduit is disclosed in U.S. Pat. No. 1,684,083 to S. C. Bloom.
Likewise, collector tubes are used to collect refrigerant in the outlet manifold header. These outlet collector tubes are intended to collect fully expanded gaseous refrigerant uniformly along their length. Since refrigerant is a gas at this point, its volume, vapor velocity, and the resulting pressure drop along the manifold or collector tube are much higher that if it remained in a liquid phase.
The increased length requirement of the manifold headers has produced increasing problems with refrigerant mal-distribution in the heat exchanger. Outlet pressure drop in the manifold headers reduces performance by both constraining refrigerant flow, inducing refrigerant flow mal-distribution, and raising the coil outlet pressure and temperature. Accordingly, there remains a need for an improved heat exchanger that provides for more uniform refrigerant distribution through out the coil.
The present invention is a heat exchanger assembly having an inlet header, an outlet header spaced apart from and substantially parallel the inlet header, and a plurality of refrigerant tubes each extending between and in hydraulic communication with the inlet header and outlet header. Contained within the outlet header is a refrigerant collector conduit adapted to provide a predetermined pressure drop (ΔP) and having a cross-section area Acollector. The refrigerant collector includes a plurality of orifices having a cumulative orifice area (nAorifice) that are spaced along the refrigerant collector. The collector conduit is in fluid communication with the outlet header for transferring the vapor phase of a two-phase refrigerant. The collector conduit cross sectional area (Acollector) and cumulative orifice area (nAorifice) are described by the equation:
wherein:
ΔP=predetermined collector pressure drop (psi);
mdot=refrigerant mass flow (lbm/min);
ρ=refrigerant density (lbm/ft3);
Acollector=cross sectional area of collector (mm2);
Aorifice=average orifice cross sectional area (mm2);
n=number of orifices;
Dorifice=average orifice diameter (mm); and
δ=collector thickness (mm).
The invention also provides a method of making a heat exchanger assembly that includes calculating the cumulative orifice area (nAorifice) and collector cross-sectional area (Acollector) when
utilizing the equation:
Accordingly, the present invention improves refrigerant distribution within a heat exchanger by increasing the cross-sectional area of the refrigerant conduit to decrease the fluid flow velocity of a refrigerant in the refrigerant conduit to thereby decrease the pressure drop along the refrigerant conduit.
Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
This invention will be further described with reference to the accompanying drawings, wherein like numerals indicate corresponding parts throughout the views. Shown in
The outlet header 30 includes an interior surface 32 that is generally cylindrical or semi-cylindrical in cross-section located between opposing outlet header end caps 35. The interior surface 32 defines an outlet header cavity 34 extending along an outlet header axis A1. Similarly, the inlet header 40 includes an inlet header interior surface 42 located between inlet header end caps 45 to define an inlet header cavity 44 extending along an inlet header axis A2. The inlet header 40 further includes an inlet 46 for receiving a two phase refrigerant and may include an inlet distributor tube (not shown) for distributing the refrigerant uniformly. The outlet header axis A1 is parallel to and substantially parallel to the inlet header axis A2; therefore, the outlet header 30 is also parallel to and substantially parallel to the inlet header 40.
Each of the headers 30, 40 includes a lanced surface 37, 47 extending between the corresponding header end caps 35, 45 and parallel to the corresponding header axis A1, A2. The lanced surfaces 37, 47 of each header are oriented toward each other and include a plurality of truncated projections 38, 48 extending into the corresponding cavity 34, 44. The truncated projections 38, 48 define a plurality of header slots 39, 49 extending transversely to the header axes A1, A2.
A plurality of refrigerant tubes 50 extend in a spaced and parallel relationship and transversely to the header axes A1, A2 between the headers 30, 40. Each of the refrigerant tubes 50 defines a fluid passage 54, and shown in
A two phase refrigerant is introduced into the inlet header 40 where the refrigerant is then uniformly distributed to the extruded tubes 50. In evaporative mode, the two phase refrigerant undergoes a liquid-to-vapor transformation as it absorbs heat from the ambient air as the refrigerant flows within the refrigerant tube 50 from the inlet header 40 to the outlet header 30. Contained within the outlet header cavity 34 is a collector conduit 70 to provide for the collection and transportation of the vapor phase of the refrigerant out of the outlet header 30.
Shown in
The plurality of orifices 76 are substantially equally spaced along the length of the collector conduit 70. As an alternative embodiment (not shown), the shape, size, and spacing of the orifices 76 can be varied along the length of the refrigerant conduit 70 to achieve uniform refrigerant distribution throughout the heat exchanger assembly 20. Shown in
A test unit was developed to evaluate the effects on refrigerant distribution of varying the cross-sectional area Acollector relative to the total orifice area nAorifice of the collector conduit. The test unit represents an operating automotive type brazed heat exchanger modified to be used as an evaporator for residential or commercial application. Key geometric variables include coil size, manifold length, and manifold area. Shown in
The thermo-graphic image 100 shows an example of a single test of the experiment where the number and size of the orifice areas are changed relative to the cross-sectional area of the collector conduit. The thermo-graphic image 100 shows an exemplary poor refrigerant distribution for test unit 110. The test unit 110 was divided into 6 approximately equal sections. An average height for each section was visually estimated and marked. The distribution metric that correlates best with flow geometry factors is the slope rating:
Since the slope can be positive or negative, it gives a directional indication of distribution. A slope equal to zero indicates perfect distribution. In other words, where the dark peaks are of equal height across the thermo-graphic image 100, the refrigerant flow is equally distributed across the test unit. It was found that the refrigerant distribution can be controlled by varying the ratio of the collector's cross sectional area (Acollector) to the collector's total orifice area (sum of the areas of the individual orifices) nAorifice.
Where Acollector is the cross-section area of the collector, Aorifice is the average open area of each orifice, and n is the number of orifices.
Refrigerant distribution has a significant effect on heat exchanger performance as it affects the percentage of frontal area that is at saturation temperature. By varying the ratio of the collector's cross sectional area (Acollector) to the collector's cumulative orifice area (sum of the areas of the individual orifices) nAorifice, different refrigerant distribution and thus different performance levels can be achieved.
As a summary, to design collector orifice pattern that controls performance loss from perfect distribution to be within 20%, 10%, and 5%, the ranges of the area ratio, respectively, are:
In the above equations Acollector is the cross-section area of the collector, and Aorifice,i is the open area of each individual orifice. The area ratio for perfect refrigerant distribution is:
It was found that the pressure drop of the refrigerant distribution system (collector) has a strong effect on heat transfer performance. The manifold and collector pressure drop increases refrigerant saturation temperature in the tubes and therefore reduces the effective temperature difference between the refrigerant and air. The pressure drop was evaluated between ports located in the center of the outlet manifold and the outlet pipe.
A theoretical performance penalty can be predicted based on refrigerant saturation pressure vs. saturation temperature relationship: for R134a the saturation curve slope is 0.778° F. sat/psi; a 5 psi pressure drop reduces ITD by 5*0.778=3.89° F.; for a 22° F. ITD test, that means a performance penalty of 3.89/22=17.7%, that is, 3.5% per psi pressure drop. Reasonable agreement was obtained between the theoretical & measured performance penalty. Given a refrigerant saturation curve slope (° F. per psi) and nominal ITD, to limit performance penalty to 20% theoretical, the collector pressure drop should be less than:
For R134a and 22° F. ITD specifically, to limit performance penalty to 20%, 15%, and 9% of theoretical, the collector pressure drop should be less than 7, 5, and 3 psi respectively.
It was found that manifold and collector geometry could be correlated with collector/manifold pressure drop. The pressure drop was evaluated between ports located in the center of the outlet manifold and the outlet pipe. An expression based on Bernoulli's equation was developed and coefficients were determined by a linear regression to the test data. The predicted pressure drop using the correlation, shown as Equation 1, is plotted against the measured pressure drop, as shown in
The correlation predicts the measured pressure drop well.
Note that pressure drop and the required collector and orifice areas to achieve a required maximum pressure drop are strongly dependant on the refrigerant flow rate and density. This means that manifold/collector design must be sized for the intended heat transfer rate and refrigerant.
For the case of optimum refrigerant distribution where
the new correlation is shown as equation 2 below:
For uniform orifice size, Equations 1 and 2 above may be used in calculating the optimal Acollector to nAorifice ratio for fabricating a heat exchanger. The method includes the steps of:
Starting with predetermined number of orifices n:
An alternative is to start with predetermined orifice area Aorifice:
It was found that smaller inlet manifold cross-section area improves performance by promoting mixing of liquid and gas refrigerant and thus improving refrigerant distribution. It was further found that the need for the collector conduit can be eliminated if the outlet manifold cross section is big enough. Shown in
To design a manifold that is large enough to work without a collector, it should be sized per the following relationship:
While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/069,221 for a MANIFOLD DESIGN FOR IMPROVED REFRIGERANT DISTRIBUTION, filed on Mar. 13, 2008, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
---|---|---|---|
61069221 | Mar 2008 | US |